What Causes Frost to Form on Sheathing?

Night-sky radiation cooling is a common and typically harmless occurrence–and the likely culprit.

I’m in the middle of constructing a woodshop/building in central Oregon, east of the Cascades (the dry side). I could not get the siding on this season, so I used a deck stain with UV block on the exterior OSB. The interior is batt insulation and drywall— no vapor barrier. It is only heated as needed with a woodstove. During certain weather conditions—cold, clear nights— there is frost on the exterior at the stud bays. What is happening, and is it OK? Will this continue to happen to the siding on an unheated building?

—JOHN C. via email

Green Building Advisor editor Martin Holladay responds: Even though I’ve never visited your construction site, I’m going to hazard a guess: It’s probable that this building is on a hillside, with no trees blocking the view on the downhill side. The frost appears on the downhill side of the building, but not the uphill side. If my guess is correct, the explanation for the frost is night-sky radiation cooling.

Frost forms on cold surfaces. If one side of your building faces the sky, the sheathing will lose heat to outer space on cold, clear nights. By morning, the sheathing is colder than the outdoor-air temperature, allowing frost to form. This is a natural phenomenon, and it’s harmless.

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